1. Field of the Invention
The present invention is related to mobile computing. In particular, the present invention is related to methods for providing an efficient infrastructure for mobile computing.
2. Discussion of the Related Art
A typical prior art wireless local area network (WLAN) is configured to provide a number of access points (APs) interconnected by and to an ethernet backbone. Each AP provides network access over a service area that is referred to as a “basic service set” (ESS) in the IEEE standard IEEE 802.111. The service areas of the APs in a typical WLAN are preferably overlapping to allow a seamless coverage and to provide capacity enhancement where the APs overlap. In a WLAN, a mobile user or station (STA) scans frequently for candidate channels to identify APs for association. Since the media access control (MAC) throughput performance under the IEEE 802.11 depends significantly on the BSS load (e.g. the number of associated stations), a strategy for selecting an AP in an effective manner is important. 1 IEEE Std. 802.11, “Wireless LAN Media Access Control (MAC) and Physical Layer (PHY) Specifications,” 1999.
In the meantime, as the use of multimedia services (e.g., voice, video, and web browsing) in a heterogeneous environment continues to proliferate, WLANs need to provide higher transmission rates and at lower costs. As each type of service has a different specific quality-of-service (QoS) requirement, WLAN standards now provide service differentiation by assigning services to different access categories and according to user priorities (see, e.g., service categories under the IEEE standard IEEE 802.11e2). Because different services may co-exist in a WLAN, an effective AP selection method should take into consideration the service type and access priority. 2 IEEE Computer Society, “Part 11: Wireless LAN Medium Access Control (MAC) and physical Layer (PHY) specifications: Medium Access Control (MAC) enhancements for Quality of Service (QoS)”, IEEE Std. 802.11e/D1.4, 2001
Various studies have been made to improve AP selection. The most common and simplest approach is based on received signal strength (RSS) measurements. An STA, upon entering the service area of a WLAN or upon detecting degradation in its existing link, scans each candidate channel and estimates the RSS of the received frames from the available APs. The goal of selecting the AP with the strongest RSS is to access a higher quality channel and at a higher data rate. This method results in a larger admission capacity in a WLAN. While an RSS-based selection method appears to provide a maximum data transmission rate to each STA, such a method ignores the contention-based nature in WLAN access. When STAs gather around one AP, an RSS-based method tends to cause traffic aggregation in some BSSs and severe frame collisions, thereby resulting in throughput degradation.
AP selection methods have also been studied as a possible efficient way to achieve load balancing among different BSSs. For example, in the article “Performance of an efficient method for association admission control in public wireless LAN,” by Hyun-woo Lee, Se-han Kim and Won Ryu, and published in the Vehicular Technology Conference (VTC), Fall, 2004, the authors suggest taking BSS load levels of the stations, and their traffic, into account in an AP selection method. Under that method, an AP inserts into one or more reserved fields of beacon or probe response frames values representing their BSS load or traffic, so as to allow the STA to select an AP with a light load and to avoid traffic congestion. Similarly, in the article “A Novel Load Sensitive Algorithm for AP selection in 4G Networks” by S. Misra and A. Banerjee, published in the International Conference of Computer and Devices for Communication (CODEC), Calcutta, India, 2003, the authors propose an association algorithm which further considers an STA's movement, in addition to the load level. The STA's direction of movement allows an AP with a light load along that direction to be selected. While these load-balancing methods may alleviate traffic imbalance between adjacent BSSs to some extent, neither the number of STAs nor the amount of traffic is an accurate measure of the load level.
The Chinese patent application “Method and Apparatus to select AP in WLAN,” serial no., 200610110741.0, filed on Aug. 8, 2007, discloses a method for AP selection which enhances total performance by taking into account hidden terminals that may cause much severe throughput degradation. Under that method, the STA avoids collisions with hidden terminals by anticipating their effects in their respective BSSs.
Other proposals present more precise load estimates to allow AP selection. For example, the article “Facilitating access point selection in IEEE 802.11 wireless networks,” by S. Vasudevan, K. Papagiannaki, C. Diot, J. Kurose, and D. Towsley, published in ACM Internet Measurement Conf., New Orleans, La., October 2005, describes a method for an STA to estimate a potential bandwidth based on delays experienced by beacon frames received from an AP. In this method, the same access priority is assumed between beacon and other data frames, so that a channel status may be predicted from an offset between the timestamp of a beacon and the target beacon transmission time (TBTT). However, using beacon delay for bandwidth estimation assumes that the beacon frames experience the same contention as other frames. In a WLAN, where the beacon frames are typically sent prior to data frames, such an assumption is typically not valid.
In addition, the article “Link Quality based Association Mechanism in IEEE 802.11h compliant Wireless LANs,” by T. Korakis et al, presented in the Workshop on Resource Allocation in Wireless Networks (RAWNET), April 2005, proposes considering the physical layer channel condition (e.g., bi-directional link rate or other channel quality metrics on the uplink and downlink) in selecting the AP to associate. However, these quality metrics (e.g., the bidirectional link rates) are available only for point coordination function (PCF) operation. Since PCF is not enabled in most WLAN products, the Korakis method has limited applications.